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Technology outlook. The use of both motor gasoline and aviation gasoline in small UA is undesirable, because it is both unsafe (JP fuels have higher flashpoints than gasoline, making them more tolerant of explosive combustion situations) and logistically difficult to support. There are currently several ongoing efforts to develop small JP5/8 fuel burning engines in the power classes and power to weight ratios being discussed here, including lightweight versions for aviation applications. For example, the opposed cylinder (OPOC) engine development program (FEV Engine Technology, Inc.) is developing a light weight, high powered diesel engine that is being sized for the A160. In addition, Nivek R&D, LLC, is developing a lightweight six-cylinder diesel engine for the A-160.
. Reliability. Reliability of current low cost two and four-cycle UA engines are on the order of a few hundred hours, sometimes less. This shortcoming, when compared to turbine engines, is often overlooked due to the low cost of reciprocating engines. However, good engine reliability has proven to be a significant factor in user acceptance of UA. Nevertheless, most UA demonstrations, and even development programs do not stress reliability in the design process, nor prove reliability in their development, many times resulting in disappointing results in extensive flight and operational testing.
APPENDIX D – TECHNOLOGIES
Page D-3
Developing reliability in a small HFE will present a large challenge due to the differences in combustion and lubrication between JP fuels and gasoline, and the duty cycles imposed on them for UA use.
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Efficiency, brake specific fuel consumption, (BSFC) and power-to-weight ratio. One of the most desirable traits for any UA is persistence, and engine fuel efficiency has a major influence on the number of UA required for a given time on target coverage. Current gasoline two cycle engines have relatively poor efficiency, while four stroke engines are better but at the cost of increased engine weight. Both engines are significantly better than small gas turbines in this power class. As a result, any effort to develop HFEs will place a large emphasis on efficiency. A HFE that operates on a true diesel cycle could double the endurance of a given UA, which normally uses a two-stroke gasoline engine. Currently, two cycle engines tend to be used extensively in small UA, particularly in demonstration efforts. They provide the UA designer a low cost and lightweight, yet powerful engine, providing significant capability per dollar. This is known as the power-to-weight ratio. Due to low cycle efficiency their BSFCs tend to be high, resulting in aircraft with limited endurance capabilities. Existing gasoline engines converted to operate on heavy fuels would not have significantly improved BSFCs, but would improve the logistics footprint by operating with a common fuel. True diesel cycle engines would offer greatly reduced BSFCs, but technological advances are required to reduce the weight of these engines to get them near the same mass as gasoline engines. The technological advances to bring two-cycle engine efficiencies up to HFE levels are equally complex.
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无人机系统路线图 Unmanned Aircraft Systems Roadmap(133)
